1. Field of the Invention
The present invention relates to a process for manufacturing magneto-optic disc.
2. Description of the Prior Art
Development of magneto-optic disc as a photo memory element in which information can be rewritten has been actively attempted in the recent year. As shown in FIG. 5, the magneto-optic disc basically comprises a transparent substrate 1 with magneto optic recording film Z provided thereon, so that laser beam or the like is irradiated in a desired recording field on the magneto-optic recording film 2 at an elevated temperature to record and erase information magnetically.
Since the magneto-optic recording film 2 is usually made of amorphous rare earth transition metal alloy and likely to be deteriorated by contacting with air, water or the likes, a protecting film 3 made of resin is usually formed on the magneto-optic recording film 2, as shown in FIG. 5, to protect the surface of the magneto-optic recording film 2. In this case, when an outer periphery 2a of the magneto-optic recording film 2 2 is exposed from the protecting film 3, the magneto-optic recording film 2 is subject to deteriorate from the outer periphery 2a. Hence, the outer periphery 2a of the magneto-optic recording film 2 is situated within approximate 4-5 mm from an outer peripheral edge 1a of the transparent substrate 1 to be also covered by the protecting film 3.
In case that the outer periphery 2a of the magneto-optic recording film 2 is covered by the protecting film 3, the magneto-optic recording film 2 is deposited, with the outer peripheral portion of the substrate 1 being supported by an annular holder 4, as shown in FIG. 6, for example by vapour deposition or spurttering from the lower part as indicated by an arrow A, so that the magneto-optic recording film 2 is formed at the surface of the substrate 1 except that supported by the annular holder 4.
FIG. 7 shows the result of measurement of reproduced signal quality C/N at specific points on radiuses of the substrate 1 of a magneto-optic disc using a polycarbonate substrate of 65 mm radius. In FIG. 7, each of plots indicated by a sign .largecircle. shows a measured value at a plain portion of the disc, and a plot indicated by a sign X shows a measured value at a groove portion of the disc. To be noted is that the plain portion is a region at an outer peripheral portion of the disc and having no groove, and the groove portion is another region provided with a groove.
As clearly seen in FIG. 7, the reproduced signal quality C/N in a region at approximately 3-5 mm from the outer periphery of the substrate 1 largely comes down due to influence by double refraction or the like. Specific measuring conditions for obtaining data shown in FIG. 7 are--rotation speed of the magneto-optic disc: 900 rpm, Laser output: 6 mW, Intensity of outside magnetic field: 300 Oe, Recording frequency: 3.14-3.35 MHz (bit length 0.9 .mu.m). Pulse duty rate of time for a practical recording per recording cycle): 17%.
As seen from the above, the magneto-optic disc using the polycarbonate substrate 1 has a lowering of reproduced signal quality C/N in a part adjacent to the outer peripheral portion. It was preferable for keeping a proper reproduced signal quality C/N that the magneto-optic recording film 2 is not provided in the region of 4-5 mm from the outer peripheral portion of the substrate 1. Even in case of a magneto-optic disc using a substrate 1 such as a glass substrate having no lowering of reproduced signal quality C/N in a part adjacent to the outer peripheral portion, it is required to have a supporting margin l of 4-5 mm for supporting the substrate 1 by the holder 4 in depositing of magneto-optic recording film 2 with the method shown in FIG. 6, so that the outer periphery 2a of the magneto-optic recording film 2 was to be situated within 4-5 mm from the outer peripheral edge 1a of the substrate 1.
The above magneto-optic disc has a problem to be disadvantageous in enlarging its memory capacity due to the fact that a magneto-optic recording film 2 is not formed in a rather wider range (approximate 4-5 mm in the radial direction) adjacent to the outer peripheral portion 1a of the substrate 1.